This invention relates generally to a valve actuator for a steam valve, and particularly to a fast opening apparatus for the valve actuator.
It is common for a number of mechanically coupled steam turbines to provide mechanical power to the rotor of an electromagnetic generator. In such a system, it is desirable to match the combined mechanical output from the steam turbines with the generator's electrical output to the load or user. In some situations, switching transients on the long-distance transmission lines from the electric power plant affect the load and, hence, change the mechanical power requirements for the rotor of the electromagnetic generator. Since a majority of the mechanical power developed by the plurality of coupled steam turbines is developed by the low pressure steam turbines (in contrast to the high pressure and intermediate pressure steam turbines), one method of quickly changing to different power levels involves utilization of an intercept valve fluidly coupled between the intermediate pressure turbine and the low pressure turbine. Generally, the intercept valve is maintained in an open position such that steam exiting the intermediate pressure turbine flows through the intercept valve to inlets of the low pressure turbine via appropriate steam admission valves. A detailed description of such an intercept valve is found in U.S. Pat. No. 3,495,501 by Kure-Jensen, which is incorporated herein by reference thereto. Simply stated, the intercept valve, which is normally open, rapidly closes to cut off steam flow between the intermediate pressure turbine and the low pressure turbine. It is to be recognized that a plurality of turbines, valves, and associated mechanical and electrical elements are included in a steam turbine electrical generation plant. However, for simplicity, only one valve will be discussed herein with reference to the present invention.
Rapid closure of the intercept valve is accomplished by rapid activation of a valve actuator mechanically connected to the valve stem of the intercept valve. In U.S. Pat. No. 3,495,501 by Kure-Jensen, the valve actuator includes a disc dump valve. The disc dump valve includes a sealing member disposed between a port in the actuator chamber and an output channel which is hydraulically coupled to a drain. The sealing member is biased against the chamber port by a spring means and by fluid pressure. Rapid closure of the valve actuator, and hence the steam valve, is accomplished by the fluid pressure and subsequently hydraulically linking the chamber port with the output port. Experimentally, steam valve actuators have been closed within approximately 0.1 seconds.
It is desirable not only to achieve rapid closure of steam valves but also rapid opening of those valves. One problem is to supply enough hydraulic fluid flow to the actuator chamber to achieve rapid opening of the steam valve. Limitations on hydraulic fluid flow supplied to the actuator chamber are sometimes caused by the control servo valve which couples a primary source of hydraulic fluid under pressure with the valve actuator chamber. Also, inability of the primary source of hydraulic fluid under all operating conditions to maintain adequate fluid pressure in the supply lines to the valve actuator as well as to the balance of the hydraulic control system may limit rapid opening of the valve actuator. It is appreciated by a person or ordinary skill in the art that hydraulic fluid under pressure is utilized extensively throughout the turbine plant to control the plurality of steam valves and hence steam turbine operation.